Generally, a cooking process has a step for soaking rice in water, a step for boiling rice, a steam--settle--down step and keeping-warm step.
Hereinafter, the step for soaking rice in water is designated by a soaking step and the step for boiling rice is designated by boiling step.
Referring to FIG. 1, this is a circuit diagram of conventional inverter cooker. The said inverter cooker comprises bottom sensor 1 positioned to the beneath of metal pan and for detecting a temperature of a metal pan which serve meal like as rice in it, comparator 2 for comparing the voltage equal to the temperature detected by said bottom sensor 1 with the reference voltage, 1st and 2nd counter 3, 4 for controlling the soaking step and a steam-settle-down step, switching part 5 which is switched according to the signal controlling cooking process, AND gate 6 which operates the signal equivalent to the switching state of the said switch part 5 with the output of the said comparator by logic product and power supply part 7 which supply a certain serial power of B+.
The said power supply part 7 includes switching element Q1 which is switched according to the output of the said AND-gate 6 and photocoupler 7a for converting a photo signal generated by switching of the said switching element Q1 to electric signal, wherein the photocoupler consist is of photodiode and phototransistor.
The detail descriptions about this conventional art are given as follows using the circuit diagram of FIG. 1.
When a cooking start key is pressed by user after power supply, system controller(not shown to the drawing) takes the soaking step. The step makes the 1st counter 3 enable and 1st switch awl a connecting state(ON state), 2nd switch SW2 a open state(OFF state). Accordingly, the said 1st counter 3 counts a system clock inputted in it.
When the soaking step starts, the bottom sensor 1 positioned to the beneath of the metal pan detects the temperature of the said metal pan. The temperature detection of the said bottom sensor 1 utilizes a characteristic which a output voltage is varied with a change in a inner resistance value due to a temperature change of the metal pan.
The output voltage from the said bottom sensor 1 is inputted to a negative terminal(-), so that it is compared with the reference voltage which is inputted to a positive terminal(+).
From the above comparation, when the voltage inputted to the negative terminal(-) is lower than the reference voltage inputted to the positive terminal(+), the output of the said comparator 2 comes to have a high level, so that it is inputted to one side input terminal of AND-gate 6 connected with its latter terminal.
In this time, because a high potential is inputted to other side input terminal of the said AND-gate 6 according to ON state of the said 1st switch SW1, the output of the AND-gate 6 is high level.
The high signal outputted from the above AND-gate 6 turns the switching element Q1 on. By means of turning the said switching element Q1 on, the serial power of +B1 is applied to the photodiode(PD) in photocoupler 7a, so that the said photodiode(PD) comes to be a state of luminescence.
The phototransistor(PT) is turned on by photo signal generated according to luminescence of the said photodiode(PD).
Accordingly, the serial electric power is applied to inverter driving part which is connected to the latter terminal, so that the inverter cooker executes the soaking step continuously. The reference voltage Vref is varied with a execution of the soaking step. For instance, the said soaking step designates its equivalent reference voltage on the basis of 58.degree. C. and the step for boiling rice on the basis of 120.degree. C.
During execution of the above step, if counting of the 1st counter 3 is ended, the said soaking step is ended, too. At this time, the system controller part(not shown to the drawing) makes the 1st switch SW1 in the switching part 5 turned off and the 2nd switch SW2 turned on, to execute a sequential step for boiling rice.
In addition, the said system controller part designates the reference voltage of comparator 2 as a voltage equivalent to cooking step temperature of 120.degree. C., and executes the above steps repeatedly, so that executes step for boiling rice.
When the boiling step is ended, it executes steam-settle-down step continuously.
The steam-settle-down step is executed by counting a time during a constant time according to the system clock like soaking step above mentioned, and as the steam-settle-down step is ended, a cooking process is ended.
Referring to FIG. 3, this is showing the state which a metal pan and bottom sensor is equipped. In case of being equipped normally like FIG. 3(A), the bottom sensor 1 can accurately detects the temperature of the bottom sensor, but in case that a needless material like rice etc. is inserted between the said metal pan A and the said bottom sensor 1, the detection temperature of the said bottom sensor is lowered relatively than that of case equipped normally like FIG. 3(B), so that it can't accurately detect the temperature of the metal pan. As cooking time is delayed pretty by means of above reason, the metal pan emits heat and surrounding temperature of the metal pan is elevated, so that there is a problem which a fire can occur.